1. Field of the Invention
The present invention relates to a method and device for adding information to an image signal, wherein the additional information is spectrally spread, superimposed at a very low level which has as little effect on a reproduced image as possible, and transmitted.
2. Description of Related Art
Devices for recording digital information, such as digital VTR and MD (mini-disc) record players, are in widespread use, and DVD (digital video disc or digital versatile disk) devices with recording functions have also appeared.
In these digital information recorders, various additional information signals can be recorded together with the main video and audio signals or computer data, etc. In this case, the additional information signal is a digital signal recorded on an area distinct from digital information signal areas, such as for example the headers added to every block of data or TOC (Table of Contents) area.
In the case of a system where additional information is transmitted by superimposing on a conventional main image signal, therefore, the additional information signal is not superimposed directly on digital information signals but is recorded on an indirect area as with headers. The additional information signal may therefore easily be lost by filtering or tampering, so that necessary additional information signals can no longer be detected by recording and playback devices. In particular, when control information and copyright information are added as additional information signals to prevent illegal duplication, their original purpose cannot be achieved due to the loss of the additional information signal.
Moreover, if the additional information signal is added to an indirect area, only the main information signal will be obtained when the digital information signal is converted to an analog signal and the additional information signal will be lost. This means that even if an anti-duplication control signal is added as an additional information signal to prevent illegal duplication, this strategy is completely ineffective when the signal is converted to an analog signal.
To resolve this problem of the disappearance of the additional information signal when the signal is converted to an analog signal, the inventors already proposed a method wherein an additional information signal such as an anti-duplication signal is spectrally spread, and the spectrally spread additional information signal is superimposed on an image signal during digital or analog recording.
In this method, spectrum spreading is performed by, for example, generating a code of a PN (Pseudorandom Noise) sequence (hereafter, referred to as a PN code) at a sufficiently early timing and applying it to the additional information signal. An additional information signal such as a narrow-band, high level anti-duplication control signal is thereby converted to a wideband, very low level signal which has no effect on the image signal. The additional information signal which has been spectrally spread in this way, i.e. the spectrally spread signal, is then superimposed on an analog image signal and recorded on a recording medium. The image signal recorded on the recording medium may be either analog or digital.
In this method, the additional information signal, such as an anti-duplication control signal, is superimposed with the same timing and frequency as the image signal. It is therefore difficult for a person who wishes to perform illegal duplication to remove the superimposed anti-duplication control signal from the image signal. However, the additional information signal such as a superimposed anti-duplication control signal can still be detected and used by performing despreading.
In this way, the anti-duplication control signal can be supplied together with the image signal to the recording side. On the recorder side, the anti-duplication control signal is detected, and anti-duplication control is performed without fail according to the detected anti-duplication control signal.
When the additional information superimposed on the image signal is reproduced, as described above, it is not removed from the signal, so it is necessary to superimpose the additional information at a minute level which does not affect the reproduced image of the image signal. However, it is important to be able to detect the spectrally spread signal from the main information signal even at a very low level, so it is desirable to increase the superimposing level as much as possible to lower the probability of incorrect detection.
Therefore, when the spectrally spread signal or additional information was superimposed on the image signal, either the effect on the reproduced image or the probability of incorrect detection had to be sacrificed to achieve an acceptable superimposing level.
The present invention therefore aims to provide a method and device for adding information which can satisfy both the requirements of decreased effect on the reproduced image and less probability of detecting the additional information incorrectly.
The present invention relates to a device for superimposing additional information, such as an anti-duplication control signal, on a video signal, generating a spectrum spreading code having chip intervals corresponding to blocks of plural pixels obtained by dividing one screen of the video signal into blocks in synchronism with the video signal, spectrum spreading the additional information by the spectrum spreading code to generate a spectrum spread additional information signal, calculating a scatter of pixel values contained in each block, generating a level control spectrum spread additional information signal for controlling the level of the spectrum spread additional information signal in a period corresponding to the aforesaid chip intervals according to the scatter, and superimposing the level control spectrum spread additional information signal on the video signal to generate an output video signal.
Human visual characteristics are such that when, for example, a minute amount of noise is superimposed on a flat image with little brightness variation, the noise is obvious, but in an image with sharp variation, the noise is not conspicuous even if it is added to the image.
In the present invention, when the image in a block is flat and there is little scatter, the superimposing level of this chip is set to a small value, whereas when the image in the block is a rapidly changing image with large scatter, the superimposing level of the block is set to a large value.
As the superimposing level of the spectrally spread signal can be increased according to scatter in this manner, although the superimposed additional information does not stand out, the probability of incorrect detection can be reduced.
That is, in the case of a flat image, the additional information superimposing amount decreases, but as described hereafter, for a flat image there is less correlation between the image and the spreading code, and as the detection sensitivity is high for this type of image as described later, the incorrect detection probability does not increase even if the superimposing level decreases. On the other hand for an image with large scatter, although the detection probability of the spectrally spread signal is poor as described hereafter, the superimposing level is set large and the superimposing amount is increased in the case of the present invention so that the detection sensitivity increases.
The relation between the scatter of the image and the detection sensitivity of the spectrally spread signal will now be described in more detail.
In the case of a signal Si comprising a spectrally spread signal superimposed on an image signal, an evaluation function xcfx86 for despreading when the spectrally spread signal is detected may be expressed as:
xcfx86=xcexa3sixc2x7pi=xcexa3(Vi+kixc2x7pi)pi=xcexa3Vixc2x7pi+xcexa3kixc2x7pixc2x7pixe2x80x83xe2x80x83(1)
Vi is a main information signal such as an image signal, pi is a spreading code such as a PN code, and ki is a coefficient.
In equation (1), the first term shows the correlation between the main information signal, i.e. an image signal etc., and the spreading code, and the second term shows the correlation of the spectrally spread signal and the spreading code.
From equation (1), it is seen that if there is no or little correlation between the main in formation signal, i.e. the image signal and the spreading code, the sensitivity of detecting the spectrally spread signal is high; conversely, when there is a large correlation between the image signal and spreading code, the sensitivity of detecting the spectrally spread signal is low.
Writing the number of chips (=blocks) per screen as N, the dispersion (equivalent to scatter) of the image as "sgr"v, dispersion of the spreading code as "sgr"p and the correlation function as xcex3, we have:
xcex3≈(1/N)xc2x7xcexa3Vixc2x7pi/("sgr"vxc2x7"sgr"p)
As "sgr"p=1,
xcexa3Vixc2x7pi≈xcex3xc2x7Nxc2x7"sgr"v
The first term in equation (1) is effectively directly proportional to the scatter "sgr"v of the image part. Therefore if the scatter of the image part is small, there is little correlation between the image signal and spreading code, the first term of equation (1) becomes small, and the detection sensitivity is high. When on the other hand there is a large scatter of the image, there is a large correlation between the image signal and spreading code, the first term in equation (1) is large, and the detection sensitivity is low.
Therefore as described above, according to the invention of claim 1, the superimposing level per chip of the spectrally spread signal is increased in parts where the sensitivity of detecting the spectrally spread signal is low, and decreased in parts where the sensitivity of detecting the spectrally spread signal is high depending on the scatter in each block, so the probability of incorrectly detecting the additional information which is the spectrally spread signal is low. Moreover, by controlling the superimposing level in this way, the additional information is prevented from standing out on the image due to human visual characteristics as described above.
According to the invention of claim 2, in the method of adding information to an image signal as claimed in claim 1, the image signal in block units is further divided into sub-blocks, and the superimposing level of additional information is controlled taking account of the scatter in each sub-block.
According to the invention of claim 2, the scatter in sub-blocks is detected, and the scatter in a block is found from the scatter in sub-block units. Hence even if there is a large local scatter in a block, a suitable scatter can still be detected for each block.
According to the invention of claim 3, in the method of adding information to an image signal as claimed in claim 1, the image signal in block units is further divided into sub-blocks, and the superimposing level per chip of the spectrally spread signal of the additional information is controlled for each sub-block in a block according to the scatter in the sub-block.
According to the invention of claim 3, the superimposing level per chip is varied according to the scatter in each sub-block. Therefore, the additional information can be superimposed more finely so that it does not stand out on the image, and the additional information can be superimposed so as to reduce the likelihood of incorrect detection. Specifically, even when the superimposing level of additional information cannot be increased at the scatter in block units, it may be possible to increase the superimposing level in sub-block units, so the sensitivity of detecting the additional information is increased overall.